Triceratops’ Quiet Cousin, the Torosaurus, Gains New Legitimacy

NEW HAVEN — Growing up in Calgary, Alberta, Daniel Field idolized John R. Horner, the well-known paleontologist who helped inspire Michael Crichton’s novel “Jurassic Park” and won a MacArthur “genius award” in 1986.

Now Mr. Field, a 23-year-old graduate student in paleontology at Yale, has found himself on the opposite side of Dr. Horner in a debate over a horned, frilled dinosaur called torosaurus — namely, whether the animal ever existed.

First identified in 1891 by the famed Yale paleontologist O. C. Marsh, torosaurus went unchallenged as a species for more than a century.

But in 2010, Dr. Horner and John Scannella, both at the Museum of the Rockies in Bozeman, Mont., published a paper arguing that torosaurus didn’t exist, at least not as a separate species. Instead, they wrote, the dinosaur was an adult version of its more famous cousin, triceratops.

That hypothesis struck Nicholas Longrich, a Yale postdoctoral fellow, as dubious. Both dinosaurs’ skulls, after all, sat side by side on display at the Peabody Museum of Natural History here at Yale, and they looked quite different. Triceratops’s bony frill is relatively short and thick. Torosaurus’s is larger and flatter, with two large circular holes called fenestrae on either side of it.

Working with Mr. Field, Dr. Longrich devised three tests to determine whether the two animals could be younger and older versions of the same species. The results, published on Wednesday in the journal PLoS One, suggest that the dinosaurs are separate animals.

The distinction may seem trivial, but it has generated much discussion in paleontology circles. It is the latest battle in what is sometimes called the war between “lumpers,” who tend to consolidate species, and “splitters,” who are more likely to tease them apart. Dr. Horner is known as one of the field’s most ardent lumpers.

“Horner’s got an agenda,” Dr. Longrich, 35, said in an interview. “He has this hit list of dinosaurs that he’s trying to get rid of.” Sitting in his lab at a desk littered with snake skeletons and empty Coke cans, he added, “Sometimes it’s fun to kind of pick a fight.”

For his part, Dr. Horner is not backing down. Last month, he took on Dr. Longrich in a lively debate at the Peabody Museum. And in December, he and Mr. Scannella published a paper pointing to fresh evidence that they said supported their hypothesis.

To test the Horner hypothesis, the Yale paleontologists came up with three hurdles for the dinosaurs to clear to be considered the same species.

First, did triceratops and torosaurus live in the same area at the same time? Yes, they concluded — in western North America in the Late Cretaceous period. But far more triceratops specimens have been discovered than torosaurus ones, an odd coincidence if the dinosaurs were both of the same species.

Second, if torosaurus was an adult version of triceratops, there should be no adult triceratops — all triceratops specimens should be less developed than all torosaurus ones.

The Yale paleontologists tested this proposition by looking at how much the specimens’ skulls had fused together — a marker of maturity, they said — and concluded that many Triceratops skulls exhibit extensive cranial fusion.

Dr. Horner does not buy it. “Bone fusion, as far as we’re concerned, doesn’t say anything,” he said in an interview, adding that the way bones fuse together has more to do with where a dinosaur died than with how old it was. A triceratops that died in a riverbed is more likely to be well preserved, with bones that look fused together; one that died on a floodplain may look less fused, as its bones had time to crumble in the sun.

At the debate last month, Dr. Horner emphasized that he and his team had unearthed more than 100 new triceratops specimens that Dr. Longrich and Mr. Field had not had the chance to examine. But in interviews, the Yale paleontologists said Dr. Horner’s published work did not cite them as evidence that they are the same species as torosaurus.

For the dinosaurs to be the same animal, Dr. Longrich and Mr. Field said, there is one more hurdle to clear. Where are the intermediate specimens, they ask, midway between triceratops, with its shorter, solid frill, and torosaurus, with holes in its frill?

In response, Dr. Horner and Mr. Scannella point to nedoceratops, a little-known horned dinosaur of which there is only one specimen. In December, the pair published a paper arguing that nedoceratops is, in effect, an adolescent version of triceratops.

In an interview, Mr. Scannella also identified another animal between the two dinosaurs: the triceratops at the American Museum of Natural History in New York City, which he said had “two pronounced depressions” in its frill near the areas where torosaurus’s holes are.

Catherine Forster, a paleontology professor at George Washington University, has analyzed the Museum of Natural History skull, however, and said she was not convinced that torosaurus and triceratops were the same animal. But she added that the limited number of fossils can make such determinations — and the science of lumping and splitting — “a very difficult thing to do.”

Peter Dodson, a University of Pennsylvania paleontologist who was a reviewer for Dr. Longrich’s and Mr. Field’s paper, said he leaned toward their findings, even though he successfully lumped several species of duck-billed dinosaurs together in the 1970s. “I’m sympathetic to the impulse to lump, to a degree,” he said, “but I think it has its limits.”

If upheld, Dr. Longrich’s and Mr. Field’s results will be a victory for the Peabody Museum, where torosaurus is something of a local hero, immortalized on posters and in a 21-foot-long bronze statue erected outside in 2005.

The dinosaur’s Yale heritage, Mr. Field said, might have influenced his and Dr. Longrich’s decision to examine it. “That’s not to say we were biased,” he said while standing near the statue. “But our results indicate that we don’t need to put up a new plaque over there. It can remain torosaurus.”

A version of this article appears in print on March 6, 2012, on page D3 of the New York edition with the headline: Triceratops’ Quiet Cousin, the Torosaurus, Gains New Legitimacy. Order Reprints|Today's Paper|Subscribe